This invention relates to an improved method for hydrolyzing and condensing dimethyldichlorosilane so as to prepare a cyclic siloxane of the general formula: EQU [(CH.sub.3).sub.2 SiO].sub.m I
where m is a whole number predominantly ranging from 3-6, inclusive. This preparation of cyclic polysiloxanes is accomplished by hydrolyzing dimethyldichlorosilane in the presence of a normal C.sub.6-16 alkyl sulfonic acid having the formula: EQU n-C.sub.x H.sub.2x+1) SO.sub.3 H II
where x=6 to 16, inclusive.
The term "normal C.sub.6-16 alkylsulfonic acid" is intended to include not only the acid itself but also its negatively charged alkali metal salts thereof, for instance, the sodium salt of the sulfonic acid. For convenience, availability, and cost, the sodium salt (which is an anionic surfactant while the corresponding free sulfonic acids are nonionic surfactants) is used in the examples which are illustrative of this invention. However, essentially all the sodium salts are analogues of the sulfonic acids and are converted to the acid in the presence of the aqueous HCl, which is used for hydrolysis purposes. Therefore, the active catalyst is the alkyl sulfonic acid, or mixtures thereof.
Among the n-alkyl sulfonic acids which can be employed in the practice of the present invention, in addition to those recited in the following examples, may be mentioned for instance, C.sub.10 H.sub.21 SO.sub.3 H, C.sub.12 H.sub.25 SO.sub.3 H, C.sub.14 H.sub.29 SO.sub.3 H, C.sub.16 H.sub.33 SO.sub.3 H, etc. Included within the scope of these sulfonic acids are the precursor alkali-metal salts thereof, such as the sodium, potassium, etc. salts, which under the reaction conditions of carrying out the invention, are converted to the sulfonic acid.
By means of this invention, the dimethyldichlorosilane when hydrolyzed gives a number of cyclic polysiloxanes according to formula I which predominantly are of the type where m is 3-6 with a minimum of linear dimethylsiloxanes terminated with silanol groups.